It’s impossible to tell, because UPKNYC won’t disclose its donors. And the mayor — once a champion of transparency for political donations — has gone radio silent.

Asked for a donors list, a spokesman said “UPKNYC is supported by individuals, organizations and foundations” committed to pre-K and after-school programs. He said the group would “fully disclose all donors and amounts beyond what is required by law,” but refused to provide their names.

Most likely, it’s the city’s public unions that are forking over the big bucks here. If so, New Yorkers ought to know — because these same unions are now negotiating contracts with the mayor. The United Federation of Teachers, for one, might also stand to gain if Albany funds a pre-K expansion that requires hiring more teachers.

How different de Blasio’s silence today is from 2010, when he blasted the Supreme Court’s Citizens United ruling: “Removing transparency, and the accountability that comes with it, have clearly made it easier for special interests to fund negative campaign ads without repercussion,” he said. “This unprecedented anonymous spending threatens voters and investors alike.”

He could have been talking about his own group. Run by many of de Blasio’s campaign aides, UPKNYC has reportedly spent more than $250,000 on the mayor’s behalf. In February, it said it would voluntarily disclose its donors in the coming weeks, but never did.

Maybe that’s because it knows if it can just hold out until the pre-K issue is resolved, the names will no longer matter.